Pictures of the new AOL Communicator strongly suggest that it is not only based on Gecko and XUL, but on Mozilla Mail as well. The UI is almost identical, the menubar has the same labels, Modern theme buttons appear on the toolbars, and they have the same features (junk mail filter, collected address book, mail headers in a grey box.)

The only interesting difference from a XUL person is the slightly different tree widget. It has buttons to scroll through the columns in place of the hide/show drop-down.